The present invention is generally directed to a battery monitor and method for monitoring the operating parameters of a battery such as a lead-acid battery. The present invention is more particularly directed to a battery monitor and method for monitoring the operating parameters of a battery and providing output signals indicative of the operating parameters such as the capacity of the battery, the relative state of charge of the battery, or the absolute state of charge of the battery independent of whether the battery is in discharge, recharge, or at rest. The battery monitor, at spaced apart time intervals, estimates the operating parameters to a high degree of accuracy based upon predetermined relationships of measured battery current, measured battery voltage, measured battery temperature, empirically determined constants, and previously determined operating parameters or measured values.
Lead-acid storage batteries have become highly refined since these devices were first utilized commercially about 125 years ago. Since its introduction, the lead-acid storage battery has distinguished itself as a highly efficient and reliable electrochemical energy source. In addition to being a highly efficient energy source, lead-acid storage batteries are also relatively insensitive to debilitating temperature effects over a broad temperature range of, for example, -40.degree. to about 160.degree. F. As a result, lead-acid storage batteries lend themselves to a broad range of utilities which continues to increase.
Transportation is one example of a commercial use for lead-acid storage batteries. Such batteries have been used for small or personal vehicle movement for quite some time. For example, such batteries have been used effectively for golf carts, wheelchairs, trolling motors or similarly small-scaled transportation devices. There has also been great interest recently in the adaptability of such storage batteries for providing power to larger vehicles such as, the so-called "electric car." Such vehicles rely heavily upon the regenerative ability of lead-acid storage batteries to provide adequate power for transportation over reasonable distances, as for example, 50 miles, without the need for recharging. For such applications, it is of obvious importance to the user that a destination be reached prior to the need to recharge or replace the batteries. Currently, because of the rarity of electric vehicles for general transportation, facilities do not exist as a general matter for recharging such batteries and hence, travel must be carefully scheduled and monitored. In turn, there is a compelling need for monitoring the state of charge of the battery or batteries to provide an accompanying fuel gauge like indication of the amount of energy remaining in the battery or batteries for continued travel.
There have been a number of proposals, in the prior art, for providing such monitoring of batteries, such as a lead-acid storage battery. For example, one arrangement examines the condition of a storage battery used as a starting, lighting and ignition battery for an internal combustion engine. This arrangement measures battery voltage under opened circuit conditions and while the battery is subjected to a predetermined AC load and a predetermined DC load. The temperature of the battery is also monitored. A microprocessor utilizes the opened circuit potential, the measured potential under both DC and AC loading, and the temperature to ascertain the characteristics of the battery. For example, internal resistance is determined and, should it be found to be excessive, the battery is deemed defective. Open circuit voltage, internal resistance and temperature provide inputs for calculating the estimated power at a fully charged state. The apparatus then discharges the battery through a reference load for about 15 seconds at constant load and measures a 15 second battery voltage. This voltage is then compared to a similar voltage of a battery at about 75% state of charge under the same conditions. If the voltage measured is higher than value maintained in a computer, the battery condition is deemed good. As a result, performance benchmarks are ascertained with a view toward determining battery capability.
Another arrangement concerns an apparatus for determining the general state of charge of a battery. This approach, however, requires the battery to be taken off-line from its circuit and connected to a calibrated resistor to determine state of charge. The process requires the battery to be subjected to two loads, one corresponding to a minimum current consumption level or load and the other a maximum consumption level or load. The process includes monitoring the load condition between the minimum and maximum load possibilities, periodically connecting the battery to a reference load when the minimum load is applied to the battery sampling the voltage across the reference load and comparing the sample voltage to an array of predetermined levels, each corresponding to a different state of charge. The comparison thereby yields an indication of the present condition of the battery.
A further arrangement determines state of charge of a battery based upon the integration of current. During the first portion of the discharge, the state of charge is evaluated by integrating the current after compensating for the rate of discharge. Later in the discharge, the state of charge of the battery is determined from the lowest subpack voltage corrected for polarization. Battery polarization is used to calculate a corrected battery voltage which is used to terminate discharge at an appropriate time.
A more improved battery monitor is shown and described in U.S. Patent No. 4,876,513 which is assigned to the Assignee of the present invention. This monitor is a dynamic state of charge indicator for a storage battery characterized by a discharge curve relating available energy of the battery to a contemporaneous voltage over a range of voltage boundaries having predetermined end points corresponding to fully charged and effectively discharged for one charge cycle. The indicator includes a microprocessor which stores predetermined relationships between the contemporaneous voltage and the state of charge of the battery. A voltage sensor and a current sensor are connected to monitor battery voltage and current flow and to provide corresponding voltage and current signals to the microprocessor. The microprocessor periodically computes the internal resistance of the battery, the average voltage and current for a predetermined discharge time period and a corrected voltage as a total of internal resistance voltage loss and average voltage with the state of charge being determined as a function of corrected battery voltage.
While the last mentioned battery monitor provided a distinct advancement in the art, there still remains a need in the art for a more effective battery monitor. For example, there is a need in the art for a battery monitor which is capable of providing indications of battery state of charge whether the battery is in discharge, recharge, or at rest. The aforementioned battery monitor disclosed in U.S. Pat. No. 4,876,513, because it relies upon a measurement of internal battery resistance, requires that the battery be in discharge. As a result, the state of charge of the battery can only be determined when the battery is in discharge. Also, because the internal resistance measurement requires a fluctuating current, such measurements are subject to inaccuracies.